A water-absorbent resin is broadly employed for sanitary products such as a disposable diaper, a sanitary napkin and an incontinence pad for adults, a soil water retention agent and the like. Such a water-absorbent resin is produced and consumed in large quantities. Examples of the water-absorbent resin encompass a partially neutralized and crosslinked polyacrylic acid, a hydrolyzed starch/acrylic graft polymer, a saponified vinyl acetate/acrylic ester copolymer, a hydrolyzed acrylonitrile copolymer or a hydrolyzed acrylamide copolymer, a crosslinked acrylonitrile copolymer or a crosslinked acrylamide copolymer, and a crosslinked cationic monomer. The above-described water-absorbent resin is produced by a method of polymerizing, under stirring, an aqueous solution including a hydrophilic monomer, the stirring crushing a polymerized gel (see Patent Literature 1), a method of statically polymerizing an aqueous solution including a monomer (see Patent Literatures 2 to 6) or like method.
Recently, a sanitary product having a reduced thickness as a result of reduction in quantity of cotton-like pulp included in the sanitary product has been commercialized. In order to produce such a sanitary product, a water-absorbent resin to be used has increased in quantity, and there has been requirement for an excellent physical property of the water-absorbent resin. Specifically, there has been requirement for an excellent water-absorption rate of the water-absorbent resin. Water-absorbent resin manufacturers have proposed various techniques (see Patent Literatures 7 to 10).
Examples of such techniques encompass a method of using a great deal of a surface active agent so as not to eliminate, prior to polymerization, bubbles suspended in a monomer solution (see Patent Literature 8), and a method of keeping a monomer solution at a low temperature (see Patent Literatures 7, 9 and 10).
However, in a case where a water-absorbent resin produced by the method of using a great deal of a surface active agent is employed for a sanitary product, the water-absorbent resin possibly causes deterioration in interfacial (surface) tension of body fluid. This interferes with a liquid-absorbent property of the sanitary product. Meanwhile, the method of keeping the monomer solution at a low temperature possibly needs long time period for polymerization. This results in poor productivity.
Examples of a foaming agent to be employed for a monomer in the above-described foam polymerization encompass carbonate (Patent Literatures 11 to 18), an organic solvent (Patent Literatures 19 and 20), inert gas (Patent Literatures 21 to 23), an azo compound (Patent Literatures 24 and 25), and insoluble inorganic powder (Patent Literature 26). Further, a technique of foaming and crosslinking after the polymerization (see Patent Literature 27) is also proposed. Furthermore, a technique of employing water-insoluble particles for polymerization (see Patent Literature 28) is also proposed.
The above-described techniques improve the water-absorption rate to some extent. However, these techniques cause, for example, deterioration in liquid permeability or impact resistance (Patent Literatures 29, and 32 to 34), or bulk specific gravity (Patent Literatures 30 and 31) of the water-absorbent resin though these properties have recently been required, or cause generation of fine powder or dust during producing (particularly, crushing) or using the water-absorbent resin. That is, these techniques are not sufficiently efficient.